British Pat. No. 1,413,488 published Nov. 12, 1978 and U.S. Pat. No. 3,673,154 indicate that the presence of iron, chromium, nickel and copper are undesirable contaminants of cobalt, or manganese, or cobalt and manganese oxidation catalyst metals used for the liquid phase oxidation of m- or p-xylene with air to the respective iso- or terephthalic acid.
In the production of those acids from the oxidation of the respective xylenes, the British Patent teaches a method of metal catalyst recovery which, for example, recovers cobalt containing no more than 120 ppm of copper, 1600 ppm of iron, 10,000 ppm of nickel and 10,000 ppm chromium on a weight basis and is said to be suitable for recycle to the oxidation without material inhibition of the foregoing xylene. The method of the British Patent comprises extracting the residue left after iso- or terephthalic acid product separation and removal of reaction solvent (e.g. acetic acid) with aqueous alkali (e.g., sodium) carbonate at a pH of 7-8 in the presence of air or up to 9.5 in the absence of oxygen which would at the higher (8 to 9.5) pH cause the formation of manganese oxide. After removal of aqueous extract solution there remains as a solid residue a mixture of carbonates of iron, manganese and cobalt. Said mixed metal carbonate residue is dissolved in 1.05 to 1.2 times the stoichiometric amount of acetic acid, preferably acetic acid containing 40 to 50 weight percent water, required to convert the metal carbonates to their acetates. Thereafter the solution is heated to distill off acetic acid until a solution pH of from 4.5 to 5.8 is reached at which pH an insoluble form of iron precipitates. Such method is illustrated as recovering 93.8% of cobalt and 97.9% of manganese (in their dissolved acetates in acetic acid) with an iron content of 0.575 ppm of the cobalt recovered from a starting residue (before aqueous sodium carbonate extraction) having an iron content of 14300 ppm of the cobalt content.
The above United States Patents heats to 93.degree. C. a residue containing acetic acid, 0.58 to 1.7 weight percent cobalt, 84 to 270 ppm of iron, 16 to 51 ppm of chromium, 13 to 40 ppm of nickel and acetic acid to evaporate acetic acid until the concentrated solution has a pH above 3. Such concentration causes a form of iron and a form of chromium to precipitate to the extent that 97% of the iron and 65% of the chromium are removed. However, this method did not remove nickel.
Recently in our laboratories, it has been found that in the neat oxidation of liquid o-xylene to o-phthalic acid can be inhibited by copper, or iron, or nickel in rather low amounts based on the cobalt metal oxidation catalyst. The oxidation retarding effects of said three metals in concentrations per million weight parts (ppm) of cobalt are shown in TABLE I to follow. The retardation effect is shown as mole percent of theoretical oxygen consumed.
TABLE 1 ______________________________________ o-XYLENE OXIDATION RETARDING EFFECT OF Cu, Fe and Ni Metals Concentration in parts Mole % Theoretical per 1 .times. 10.sup.6 parts of Co Oxygen Consumed ______________________________________ Cu, 80 ppm 113 Cu, 800 ppm 67 Cu, 8000 ppm 65 Cu, 9100 ppm 42 Cu, 80,000 ppm 28 Fe, 500 ppm 116 Fe, 5000 ppm 103 Fe, 50,000 ppm 70 Ni, 6700 ppm 112 Ni, 67,000 ppm 68 ______________________________________
Thus for such neat oxidation of liquid o-xylene with air there should not be present in the original or recycle catalyst an amount of copper in the range of from 80 up to 8000, or of iron in the range of from 5000 up to 50,000, and/or of nickel in the range of from 6700 up to 67,000 weight parts per million weight parts of cobalt.
The problem left by the prior art is how to remove the nickel to the safe level of about 6700 ppm, that is, a nickel content of not more than 6700 ppm of the cobalt and to remove the copper to a safe level of about 80 ppm, that is, not more than 100 ppm, preferably not more than 80 ppm, of the cobalt.